Silazanes, which have a Si--N--Si bond configuration, are increasingly important because they can be pyrolyzed to yield ceramic materials, such as silicon carbide and silicon nitride.
Silazanes are usually synthesized by an ammonolysis process wherein ammonia or a primary amine is reacted with a halide substituted silane. The ammonolysis of organohalosilanes is a complex process consisting of several concurrent reactions as shown below. These formulas carry no structural implication, they merely are average formulations to illustrate the reactions such as: ##STR1##
and Homo- and heterofunctional condensation. ##STR2##
The preparation of silazanes by ammonolysis has been described in several U.S. patents. For instance, U.S. Pat. No. 4,395,460, issued to Gaul, describes a process for the preparation of polysilazanes in which gaseous ammonia is introduced to a solution of chlorodisilanes that have been dissolved in an inert solvent. However, during the reaction NH.sub.4 Cl is precipitated concurrently with the formation of the ammonolysis products. The precipitated NH.sub.4 Cl greatly increases the viscosity of the reaction mixture and interferes with the progress of the reaction. To overcome this problem, additional inert solvent must be added to the reaction mixture to facilitate agitation of the mixture. Furthermore, to recover a purified ammonolysis product several constituents of the reaction product mixture have to be removed. The precipitated NH.sub.4 Cl formed during the reaction and intermixed with the ammonolysis products has to be removed by filtration and the filter cake washed with additional solvent for complete product recovery. Subsequently the inert solvent which is used for dissolving the chlorodisilanes, for reducing the viscosity of the reaction mixture, and for washing the filtered crystals must be removed from the preferred products.
U.S. Pat. No. 4,954,596, issued to Takeda et al, describes preparation of organosilazanes by introducing gaseous ammonia into a reaction mixture comprising organochlorosilanes dissolved in an organic solvent. However, the added organic solvent must be removed by distillation to isolate the silazane products. Likewise in U.S. Pat. No. 2,564,674, organochlorosilanes are dissolved in ether before the ammonolysis process and additional ether is added during the process to dissolve the silicon compounds and prevent their gelation. Again, purification of the final product requires several steps.
U.S. Pat. No. 4,255,549, issued to Christophliemk et al., describes reacting organohalosilanes, dissolved in an inert solvent, with liquid ammonia to form ammonolysis products. To maintain the reaction course and to prevent overheating due to a high heat of reaction and/or heat of crystallization of precipitating ammonium halide salt, an inert solvent is added to the reaction vessel. As a result of this addition, the solvent has to be evaporated under controlled conditions to produce the polymer films.
As apparent from the foregoing description, preparing silazane products by known ammonolysis methods leads to unwanted co-products, such as NH.sub.4 Cl precipitates, that prompts the need for increased additions of inert solvent to the reaction mixture. The addition of the solvent is required to decrease the viscosity and improve agitation of the reaction slurry. Furthermore, an inert solvent is needed to reduce the heat of reaction and/or heat of crystallization due to precipitating ammonium halide salts. However, the NH.sub.4 Cl precipitates must be filtered from the reaction slurry and the inert solvent removed from the final ammonolysis product.
Another problem encountered during the production of silazanes is the formation of a high proportion of low molecular weight species. These low molecular weight silazanes can evaporate during pyrolysis resulting in a reduced weight yield of the ceramic product relative to the starting silazane material. British patent, 737,229, issued to Midland Silicones Limited, describes a method for producing silazanes wherein organohalosilanes, completely substituted with organic groups and/or halogen atoms and dissolved in an inert solvent, are added simultaneously to ammonia under pressure. However, the majority of prepared organocyclosilazanes are limited by the starting compounds to only 3-4 Si--N linkage units and a low yield of polysilazanes. As such, the prepared silazanes are volatile and difficult to pyrolize to ceramic material.
Accordingly, there is a need for novel silazanes and/or polysilazanes having an increased number of Si--N units and for improved methods for preparing silazanes, and/or polysilazanes that provide a means to easily separate desired products from any unwanted co-products generated in the reaction, that do not require large quantities of inert solvent to be introduced into the reaction mixture, that moderate the reaction exotherm for quick and efficient ammonolysis and provide polysilazanes having an increased number of Si--N linkages.